Glaucoma is one of the leading causes of blindness. Approximately 15% of cases of blindness world-wide result from glaucoma. The most common type, primary open-angle glaucoma, has a prevalence of 1/200 in the general population over 40 years of age. Glaucoma has been simply defined as the process of ocular tissue destruction caused by a sustained elevation of the Intra Ocular Pressure (IOP) above its normal physiological limits. Although several etiologies may be involved in the glaucoma complex, an absolute determinant in therapy selection is the amount of primary and/or induced change in pressure within the iridocorneal angle.
Current therapies include medications or surgeries aimed at lowering this pressure, although the pathophysiological mechanisms by which elevated IOP leads to neuronal damage in glaucoma are unknown. Medical suppression of an elevated IOP can be attempted using four types of drugs: (1) the aqueous humor formation suppressors (such as carbonic anhydrase inhibitors, beta-adrenergic blocking agents, and alpha2-adrenoreceptor agonists); (2) miotics (such as parasympathomimetics, including cholinergics and anticholinesterase inhibitors); (3) uveoscleral outflow enhancers; and (4) hyperosmotic agents (that produce an osmotic pressure gradient across the blood/aqueous barrier within the cilliary epithelium). A fifth category of drugs, neuroprotection agents, is emerging as an important addition to medical therapy, including, for example, NOS inhibitors, excitatory amino acid antagonists, glutamate receptor antagonists, apoptosis inhibitors, and calcium channel blockers.
Reviews of various eye disorders and their treatments can be found in the following references: Bunce et al., 2005, Graefes Arch Clin Exp Opthalmol.; 243(4):294; Costagliola et al., 2000, Exp Eye Res. 71(2):167; Costagliola et al., 1995, Eur J. Opthalmol., 5(1):19; Cullinane et al., 2002, Br J Opthalmol., 86(6):676; Sakaguchi et al., 2002, Curr Eye Res. 24(5):325; Shah et al., 2000, J Cardiovasc Pharmacol., 36(2):169, and Wang et al., 2005, Exp Eye Res., 80(5):629.
RNA interference refers to the process of sequence-specific post-transcriptional gene silencing mediated by short interfering RNAs (siRNA). After the discovery of the phenomenon in plants in the early 1990s, Andy Fire and Craig Mello demonstrated that double-stranded RNA (dsRNA) specifically and selectively inhibited gene expression in an extremely efficient manner in Caenorhabditis elegans (Fire et al., 1998, Nature, 391:806). The sequence of the first strand (sense RNA) coincided with that of the corresponding region of the target messenger RNA (mRNA). The second strand (antisense RNA) was complementary to the mRNA. The resulting dsRNA turned out to be several orders of magnitude more efficient than the corresponding single-stranded RNA molecules (in particular, antisense RNA).
The process of RNAi begins when the enzyme, DICER, encounters dsRNA and chops it into pieces called small-interfering RNAs (siRNA). This protein belongs to the RNase III nuclease family. A complex of proteins gathers up these RNA remains and uses their code as a guide to search out and destroy any RNAs in the cell with a matching sequence, such as target mRNA (see Bosher & Labouesse, 2000, Nat Cell Biol, 2000, 2(2):E31, and Akashi et al., 2001, Antisense Nucleic Acid Drug Dev, 11(6):359).
In attempting to utilize RNAi for gene knockdown, it was recognized that mammalian cells have developed various protective mechanisms against viral infections that could impede the use of this approach. Indeed, the presence of extremely low levels of viral dsRNA triggers an interferon response, resulting in a global non-specific suppression of translation, which in turn triggers apoptosis (Williams, 1997, Biochem Soc Trans, 25(2):509; Gil & Esteban, 2000, Apoptosis, 5(2):107-14).
In 2000 dsRNA was reported to specifically inhibit 3 genes in the mouse oocyte and early embryo. Translational arrest, and thus a PKR response, was not observed as the embryos continued to develop (Wianny & Zernicka-Goetz, 2000, Nat Cell Biol, 2(2):70). Research at Ribopharma AG (Kulmbach, Germany) demonstrated the functionality of RNAi in mammalian cells, using short (20-24 base pairs) dsRNA to switch off genes in human cells without initiating the acute-phase response. Similar experiments carried out by other research groups confirmed these results. (Elbashir et al., 2001, Genes Dev, 15(2):188; Caplen et al., 2001, Proc. Natl. Acad. Sci. USA, 98: 9742) Tested in a variety of normal and cancer human and mouse cell lines, it was determined that short hairpin RNAs (shRNA) can silence genes as efficiently as their siRNA counterparts (Paddison et al, 2002, Genes Dev, 16(8):948). Recently, another group of small RNAs (21-25 base pairs) was shown to mediate downregulation of gene expression. These RNAs, small temporally regulated RNAs (stRNA), regulate timing of gene expression during development in Caenorhabditis elegans. (for review see Banejee & Slack, 2002 and Grosshans & Slack, 2002, J Cell Biol, 156(1):17).
Scientists have used RNAi in several systems, including Caenorhabditis elegans, Drosophila, trypanosomes, and other invertebrates. Several groups have recently presented the specific suppression of protein biosynthesis in different mammalian cell lines (specifically in HeLa cells) demonstrating that RNAi is a broadly applicable method for gene silencing in vitro. Based on these results, RNAi has rapidly become a well recognized tool for validating (identifying and assigning) gene function. RNAi employing short dsRNA oligonucleotides will yield an understanding of the function of genes that are only partially sequenced.
The preceding is a discussion of relevant art pertaining to RNAi. The discussion is provided only for understanding of the invention that follows, and is not an admission that any of the work described is prior art to the claimed invention.